1. Field of the Invention
The present invention relates to a multiplex laser light source, and more particularly to a multiplex laser light source that multiplexes laser beams emitted from a plurality of semiconductor lasers by making use of optical fibers. The invention also relates to an exposure apparatus which employs the multiplex laser light source as the exposure light source.
2. Description of the Related Art
As conventional devices that can emit a laser beam in an ultraviolet region, a wavelength conversion laser (which converts infrared light, emitted from a solid-state excitation laser, to a third harmonic in an ultraviolet region), an excimer laser, and an argon-ion (Ar) laser have been put to practical use.
Furthermore, a GaN semiconductor laser has recently been provided as a device that can emit a laser beam having a wavelength near 400 nm (see Jpn. Appl. Phys. Lett., vol. 37, 1998, p. L1020).
It is also conceivable that the light source to emit a laser beam of such wavelengths can be applied as an exposure light source for an exposure apparatus that exposes a photosensitive material which has sensitivity in a predetermined wavelength region including an ultraviolet region of 350 to 420 nm (hereinafter referred to as an ultraviolet region). The exposure light source in that case is required to have enough output to expose the photosensitive material.
The aforementioned excimer laser, however, has the problem that it is bulky and costly and has a high maintenance cost.
In addition, the aforementioned wavelength conversion laser, which converts infrared light to a third harmonic in an ultraviolet region, is very low in wavelength conversion efficiency, so it is extremely difficult to obtain high output. At present, a solid-state medium is excited with a semiconductor laser of 30 W to emit a 10-W fundamental wave of wavelength 1064 nm; the fundamental wave is converted to a 3-W second harmonic of wavelength 532 nm; and a 1-W third harmonic of wavelength 355 nm, which is the sum frequency between them, is obtained. The electro-optic efficiency of the semiconductor laser in that case is on the order of 50%, and the efficiency of converting infrared light to ultraviolet light is very low, typically on the order of 1.7%. Such a wavelength conversion laser is considerably costly because it employs an expensive wavelength conversion element.
Furthermore, the aforementioned Ar laser has the problem that the electro-optic efficiency is very low (0.005%) and the lifetime is very short (on the order of 1000 hours).
On the other hand, for the aforementioned GaN semiconductor laser, a GaN crystal substrate in a low dislocation region is not obtained. Because of this, a low dislocation region on the order of 5 xcexcm is formed by a growth method (ELOG), and a laser region is formed on the low dislocation region to achieve high output and high reliability. However, even in the GaN semiconductor laser fabricated in this manner, it is difficult to obtain a low-dislocation substrate over a large area, and consequently, GaN semiconductor lasers with a high output of 500 mW to 1 W have not been put to practical use yet.
As another method for obtaining high-output semiconductor lasers, it is conceivable to obtain an output of 10W by forming 100 cavities which each have a light output of 100 mW. However, it is extremely difficult to produce such a great number of cavities in high throughput. Particularly, for GaN semiconductor lasers, in which a throughput of 99% or greater is difficult even in the case of a single cavity, it is still more difficult to produce a great number of cavities in high throughput.
The present invention has been made in view of the aforementioned circumstances. Accordingly, it is an object of the present invention to provide an inexpensive multiplex laser light source in which high output is obtained. Another object of the invention is to provide an exposure apparatus that is capable of exposing photosensitive materials with high-intensity laser light by employing the multiplex laser light source mentioned above.
To achieve the aforementioned objects of the present invention, there is provided a multiplex laser light source comprising:
a plurality of semiconductor lasers;
a single multi-mode optical fiber; and
a light-collecting optics system for collecting laser beams emitted from the plurality of semiconductor lasers and then coupling the collected laser beams to the multi-mode optical fiber.
In a preferred form of the present invention, the aforementioned plurality of semiconductor lasers are disposed so that their light-emitting points are arranged in a row in a first direction parallel to their active layers. The aforementioned light-collecting optics system comprises a plurality of collimator lenses, each having a first aperture diameter in the first direction and a second aperture diameter larger than the first aperture diameter in a second direction perpendicular to the first direction, and provided so that they correspond to the plurality of the semiconductor lasers. The light-collecting optics system further comprises a collective lens for collecting the plurality of laser beams collimated by the plurality of collimator lenses and then converging the collimated laser beams on an end face of the multi-mode optical fiber.
In another preferred form of the present invention, the aforementioned plurality of collimator lenses are formed integrally with one another and are constructed as a lens array. In addition, a block on which the aforementioned plurality of semiconductor lasers are mounted is divided into a plurality of subblocks, and the subblocks are bonded with one another.
In still another preferred form of the present invention, the aforementioned plurality of semiconductor lasers comprise 3 to 10 semiconductor lasers arranged in a row. It is further preferable that the plurality of semiconductor lasers comprise 6 or 7 semiconductor lasers arranged in a row. Each semiconductor laser has a light-emitting width of 1.5 to 5 xcexcm, preferably 2 to 3 xcexcm. It is desirable that the semiconductor lasers be GaN semiconductor lasers.
In the multiplex laser light source of the present invention, it is desirable that the aforementioned multi-mode optical fiber have a core diameter of 50 xcexcm or less and a numerical aperture of 0.3 or less. It is further desirable that the value of (core diameterxc3x97numerical aperture) of the multi-mode optical fiber be 7.5 xcexcm or less.
In the multiplex laser light source of the present invention, it is desirable that a plurality of semiconductor lasers be arrayed and fixed two-dimensionally when viewed from a side where the laser beams are received.
The multiplex laser light source of the present invention may employ only a single multi-mode optical fiber. However, it is desirable that the multiplex laser light source employ a plurality of multi-mode optical fibers. In this case, each of the plurality of multi-mode optical fibers may be combined with a plurality of semiconductor lasers and a light-collecting optics system so that a high-output laser beam is emitted from each multi-mode optical fiber. In such a case, it is desirable that at least the exit end portions of the multi-mode optical fibers be disposed in one-dimensional array form or bundle form.
In accordance with the present invention, there is provided an exposure apparatus having a light source. The light source comprises the aforementioned multiplexer laser light source in which a plurality of multi-mode optical fibers are disposed in one-dimensional array form or bundle form.
According to the multiplex laser light source of the present invention, laser beams emitted from a plurality of semiconductor lasers are collected and are then coupled to the multi-mode optical fiber. Thus, the multiplex laser light source of the present invention is extremely simple in construction. Particularly, since the multiplex laser light source does not require elements difficult to fabricate, it can be formed at low cost.
According to the multiplex laser light source of the present invention, a plurality of semiconductor lasers are disposed so that their light-emitting points are arranged in a row in a first direction parallel to their active layers. In addition, the light-collecting optics system is constructed of (1) a plurality of collimator lenses, each having a first aperture diameter in the first direction and a second aperture diameter larger than the first aperture diameter in a second direction perpendicular to the first direction, and provided so that they correspond to the plurality of the semiconductor lasers, and (2) a collective lens for collecting the plurality of laser beams collimated by the plurality of collimator lenses and then converging the collimated laser beams on an end face of the multi-mode optical fiber. With this constitution, the pitch between the semiconductor lasers can be made shorter and the semiconductor lasers can be disposed in higher density. If a plurality of semiconductor lasers are disposed in higher density, the positional shift of laser beams at the end face of the optical fiber can be reduced so that it becomes smaller. Therefore, there is obtained an advantage that the plurality of semiconductor lasers, the multi-mode optical fiber, and the light-collecting optics system can be assembled with relatively low accuracy. Because of this, the number of laser beams that are multiplexed can be increased to obtain higher output. The reason for this will be described in detail along embodiments to be described later.
According to the multiplex laser light source of the present invention, a plurality of collimator lenses are formed integrally with one another and are constructed as a lens array. Therefore, formation of a large ineffective region in the circumferential portion of each lens can be avoided. As a result, the lenses can be disposed in close proximity to one another. Because of this, a plurality of semiconductor lasers can be disposed in even higher density. Therefore, the effect of being able to reduce positional accuracy, and the effect of being able to obtain high output by increasing the number of laser beams that are multiplexed, become more conspicuous. Furthermore, alignment of the collimator lenses with respect to the multi-mode optical fiber is simplified because all that is required is to make adjustments to a single lens array.
In the fields of printing and medical imaging, or in the case where an image, obtained by a printed circuit board (PCB), a plasma display (PDP), a liquid crystal display (LCD), etc., is exposed and recorded on photosensitive material, a laser spot becomes finer and therefore a high-resolution image can be exposed, if a multi-mode optical fiber with a core diameter of 50 xcexcm or less is used. In addition, if the multi-mode optical fiber has a numerical aperture (NA) of 0.3 or less, enough focal depth to expose a high-fine image is assured and therefore it becomes possible to expose an image having high sharpness.
According to the multiplex laser light source of the present invention, the value of (core diameterxc3x97numerical aperture) of the multi-mode optical fiber is 7.5 xcexcm or less. As the value, there are 50 xcexcmxc3x970.15, 40 xcexcmxc3x970.188, 30 xcexcmxc3x970.25, 25 xcexcmxc3x970.3, etc. If a multi-mode optical fiber with such a characteristic is employed, a laser beam from each semiconductor laser can be collimated with a collimator lens of the same NA as the aforementioned NA. In addition, a multiplexed laser beam can be collected to a spot of 25 xcexcm or less, with a collective lens having an NA of 0.3. This makes it possible to assure high resolution and enough focal depth.
According to the multiplex laser light source of the present invention, a block on which a plurality of semiconductor lasers are mounted is divided into a plurality of subblocks, and the subblocks are bonded with one another. Therefore, the mounting rate can be enhanced, compared with the case where semiconductor lasers are all mounted on a single block. For example, in the case where all 6 semiconductor lasers are mounted on a single block when a mounting rate for a single semiconductor laser is 98%, the total mounting rate is 86% (=0.986xc3x97100). On the other hand, in the case where 3 semiconductor lasers are respectively mounted on two blocks, the total mounting rate is 94% (=0.983xc3x97100) because a joining rate for two blocks is approximately 100%.
According to the multiplex laser light source of the present invention, three or more semiconductor lasers are provided. In conventional multiplexing of polarized beams, only laser beams emitted from two semiconductor lasers can be multiplexed. On the other hand, in the present invention, a multiplexed beam having an output which is higher than that of the conventional case can be obtained. However, in the case where 10 semiconductor lasers are provided when a mounting rate for a single semiconductor laser is 98%, the mounting rate is reduced to 82%. Since a further reduction in the mounting rate must be avoided, it is preferable that the number of semiconductor lasers that can be multiplexed be 10 or less.
In the case where 10 semiconductor lasers are disposed in a row, the mounting accuracy required is extremely high (less than 0.1 xcexcm) when using a multi-mode optical fiber having a core diameter of 50 xcexcm or less and an NA of 0.3 or less, or having (core diameterxc3x97NA) of 7.5 xcexcm or less. However, in the case of 6 or 7 semiconductor lasers, the mounting accuracy required is considerably reduced (less than 0.3 to 1 xcexcm). In addition, in the case of 6 or 7 semiconductor lasers, a high output which is equal to at least two times the output of the case of 3 semiconductor lasers can be obtained.
According to the multiplex laser light source of the present invention, each semiconductor laser has a light-emitting width of 1.5 to 5 xcexcm and is constructed of a GaN semiconductor laser. As a result, high output (50 mW or greater) can be obtained, compared with the maximum output that is obtained by semiconductor lasers having a perfect single transverse mode structure. In addition, in the multiplex laser light source of the present invention, each semiconductor laser may have a light-emitting width of 5 xcexcm or less. In this case, a light collecting-coupling system comprising 3 or more semiconductor lasers can be constituted with respect to a multi-mode optical fiber having (core diameterxc3x97NA) of 7.5 xcexcm or less. Furthermore, in the multiplex laser light source of the present invention, each semiconductor laser may have a light-emitting width of 2 to 3 xcexcm. In this case, in the aforementioned image forming system a light collecting-coupling system comprising 6 or 7 semiconductor lasers can be constituted.
According to the multiplex laser light source of the present invention, a plurality of semiconductor lasers are arrayed two-dimensionally when viewed from a side where the laser beams are received. Therefore, since a great number of semiconductor lasers can be disposed in high density, a great number of laser beams are incident on a single multi-mode optical fiber. As a result, a multiplexed laser beam with higher output can be obtained.
According to the multiplex laser light source of the present invention, at least the exit end portions of the multi-mode optical fibers are disposed in one-dimensional array form or bundle form. Therefore, high-output laser beams can be emitted one-dimensionally or two-dimensionally from the optical fibers. If each of the laser beams emitted one-dimensionally or two-dimensionally is caused to be incident on each modulating portion of a space light modulation element, such as a GLV or DMD where the modulating portions are arrayed in a row or two-dimensionally, the laser beams can be efficiently modulated for image exposure, etc.
Hence, in the exposure apparatus of the present invention with the aforementioned multiplex laser light source as its light source, the space light modulation element is employed so that laser beams emitted two-dimensionally can be irradiated to photosensitive material two-dimensionally. Alternatively, laser beams emitted one-dimensionally or two-dimensionally are irradiated to photosensitive material, and the photosensitive material is moved in a sub or horizontal scanning direction with respect to the laser beams. In this manner, a two-dimensional image can be recorded on the photosensitive material.